Defective bone remodeling is the pathophysiologic basis of most metabolic bone diseases, including post-menopausal and age-dependent osteoporosis. During the tenure of this project, we have demonstrated hat osteoblasts are coupled through functional gap junctions, formed primarily by the gap junction protein, connexin43 (Cx43), and also by connexin45 (Cx45) [new gene designations: Gja1 and G/a7, respectively]. The finding that Gja1 mutations cause the human disease oculodentodigital dysplasia (ODDD), characterized primarily by skeletal abnormalities, further demonstrates that the skeleton represents one of :he main sites of action of Cx43. Indeed, we find that conditional G/a1 deletion in osteoblasts results in significant osteopenia and reduced bone formation rates, and that Cx43 is required for a full response to anabolic signals. The central hypothesis of this renewal application is that osteoblast connexins, Cx43 and Cx45, are essential modulators of skeletal growth and osteoblast function, and are involved in homeostatic responses to hormonal and physical stimuli in the post-natal skeleton. To test this hypothesis, we propose to determine;1: the relative contribution of Cx43 and Cx45 to postnatal skeletal growth and maintenance;2: the cellular and molecular mechanisms of connexin regulation of osteoblast differentiation and function: 3: the role of connexins in the homeostatic response to mechanical load in the postnatal skeleton. The proposed studies will take advantage of novel models of conditional Gja1 and G/a7 deletion, as well as of a new Gja1 ODDD mutant, building upon experimental methods and expertise we have accumulated during the tenure of this grant, and take the experimentation on gap junction biology in bone to a more translational level. This research will study two molecules that allow bone cells to directly communicate with each other thus influencing each others'function and ability to manufacture new bone. Results will allow us to better understand how the skeleton grows and becomes denser after birth, and how these molecules influence the skeletal response to disuse and mechanical load. The results will disclose new mechanisms by which bone is maintained in post-natal life, thus helping devise new therapeutic approaches to prevent bone loss and reduce fracture risk.